Field of the Invention
The present invention relates to a protective device for subassemblies, for example subassemblies having electronic components.
Integrated semiconductor circuit known as “chips” are generally platelets based on-silicon or gallium arsenite and cut out of a wafer with electric components produced photolithographically or by similar methods on their surface. In the use of integrated semiconductor circuits, the question of effective protection against mechanical and/or chemical influences is often posed. This question is often answered by introducing the chip into a housing, normally produced from plastic. However, a weak point in the chip mounting lies in the reaction of the electrical contacts between chip and a substrate accommodating the chip, such as a circuit board, to the action of forces on the chip. In the event of displacement of the component or else in the event of direct action on the contact elements, the latter can be damaged. As a result of the use of housings, the problem is merely displaced from the actual chip contacts (for example thin wires or contact elements similar to balls and ball-grid arrays) to the contacts of the housing, even though the latter can normally be more highly loaded mechanically. In principle, however, the contact elements still have too low a resistance against forces acting on the component (i.e., the chip).
In recent times, reasons regarding fabrication, space, and economy, manufacturing increasingly utilizes “flip-chips”. Flip chips are integrated semiconductors fixed directly to the substrate, i.e., the circuit board, without a housing and contacted electrically. For this purpose, the individual chips still in the wafer composite are provided with suitable flexible contacts and, after the wafer has been cut, are placed on the substrate with the contact side down. In order to permit testing of the flip-chips before they are used, customary test devices are used. In order to be able to use such test devices on the wafer, the contacts have to be constructed to be sufficiently compliant that they are able to compensate for tilting between the surface of the wafer in the area of the flip-chip to be tested and the test device. However, after the flip-chip has been mounted on the substrate, this necessary compliance leads to corresponding mechanical sensitivity and to an increased need for protective measures.
A further development in recent times has been microcircuit boards, as they are known, in which a plurality of chips are applied to the microcircuit board substrate with the structured sides upward and are connected to the conductor track structures of the microcircuit boards by wire bonding. Here too, protection of the chips and in particular of the contact wires as well against mechanical effects is desirable.
Conventional fixings utilize a mechanical protective element such as a covering placed onto a substrate, for example, a rivet. The tolerances of the substrate, the receiving holes for the rivets, and the protective element allow movements of the protective element in the X and Y direction after mounting. Movements in the X and Y direction mean parallel to the plane of the substrate. However, in microelectronics, the dimensions of the components are preferably reduced. For this reason, the underside of the coverings comes closer and closer to the actual chip. Bringing them closer can lead to the mechanical problems described above. In particular, this can lead to damage to the component in the case of sensitive components with flexible interconnect elements.
Such a protective element is generally also used as a heat spreader. Here, the components located underneath, for example semiconductor components, are coupled mechanically and thermally by a thermally conductive material. The material is known in the art as the “gap filler” of the protective element. A gap filler can include silicon, for example, and can contain a proportion of metals or metal oxides. As a result, movement of the “heat spreader” is transmitted directly to the components. Excessive movement of the protective element can damage the electrical connections and can lead to failure of components. As result of the often low inherent stiffness of the protective element, protection of the components located underneath against compression in the Z direction, that is to say perpendicular to the plane of the substrate, is likewise not ensured.
The prior art proposes a solution to the problem of the unintentional movement of the protective element. According to one solution, the protective element is joined to the substrate by very accurately fitting riveted connections. This has the disadvantage that the selection of suitable rivets is very restricted and/or a high degree of accuracy of substrate and covering is required. Other fixing variants that could ensure good fixing often cannot be implemented because of the dimensional relationships of a subassembly or the surroundings into which this subassembly is to be integrated.
A further possibility is to bond the covering adhesively to the substrate. However, this results in an additional process step and therefore increases the costs. Compression protection in the Z direction has hitherto not been carried out or only by supporting the components in the Z direction. An example of supporting the components is adding a full or partial underfill under the components. Such an underfill projects under the components, requires a great deal of work, and is subject to faults.